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A few years ago, Jarvis and his colleagues made the surprising discovery that when a
songbird, parrot or hummingbird is producing its learned vocalization, a set of seven similar
structures in the birds brains become active. The finding was unexpected because the
three avian groups are only distantly related to one another. At the same time, they are
closely related to other birds that are not vocal learners. Flycatchers, for example, belong to
the same order as songbirds—Passeriformes—yet no flycatcher species tested so far displays
the trait.
One possible explanation, says Jarvis, is that the three groups of vocal learning birds
had a common ancestor that possessed the skill. But this means there would have been
multiple losses of the ability over time, a sort of mass extinction of vocal learning,” he says.
Another hypothesis is that vocal learners evolved similar brain structures independently
over the last 65 million years, much the same way that birds and bats separately evolved
wings that turned out to be so much alike.
Discoveries about the human brain support this latter hypothesis. Scientists conducting
imaging studies have found that when people speak, parts of their brains cerebrums that
are similar to those of vocalizing songbirds, parrots and hummingbirds become active.
They’ve also found that the same neural pathways are damaged in people who have lost the
ability to speak due to injury or stroke. Jarvis now believes that vocal learning most likely
developed independently in humans and the three bird groups (as well as in other learners
whose brains have not been studied)—yet it arose from a preexisting brain system, probably
shared by all vertebrates, that controls learning to move. 楼主~请问songbirds发声是类似这篇吗 |
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